China has abundant rare earth resources. Rare earth ores in Baiyunebo in Baotou account for 80% of rare earth resources in China, besides bastnaesite ores, monazite ores, etc. These rare earth resources are mostly light rare earths, while ion-adsorption type rare earth ore of South China is rich in heavy rare earths. The rare earth ores are also associated with radioactive thorium, which can be brought into rare earth waste residues in the smelting process. Thus it causes not only a lot of waste of resources, but also environmental pollution.
A method for separating valuable components from monazite slag, disclosed in Chinese Patent No. CN201210489542.0, used 25-35 wt % P204 (di(2-ethylhexyl)phosphoric acid) to extract thorium from a feed solution, which was leached with sulfuric acid. After that, the solution was back-extracted with a 3.0 mol/L sodium hydroxide solution to give a solid thorium hydroxide product. Because the back-extraction process was under the strong bases environment in the method, the solubility of extractant was dramatically increased, causing a significant loss of the extractant.
A method for recovering rare earth elements from acid-soluble slag of ion-type rare earth smelting was disclosed in Chinese Patent No. CN201310052866.2. After leached with acids, rare earth elements were precipitated with oxalic acid. However, it was difficult for thorium to be effectively separated because of co-precipitation effect.
Therefore, recently a new method of extraction and separation is needed to recover high-purity thorium and rare earth elements from rare earth waste residues.